Sparks die down, the arc goes dark, and suddenly the molten pool starts to cool—that’s the moment every welder needs to understand when working with GMAW. Stop the arc too early, and the weld may not fuse properly.
Leave it too long, and you risk excess spatter, burn-through, or a weak joint. I’ve learned this the hard way—trial and error on real jobs taught me that the moments after you lift the gun are just as important as the weld itself.
Knowing what happens when the welder stops the GMAW arc isn’t just theory. It affects weld strength, bead appearance, and even safety in the workshop. Let me walk you through the critical steps and consequences so you can finish every weld cleanly and consistently.
Image by setxind
What Physically Happens the Instant You Stop the GMAW Arc
The moment you release the trigger, several things occur in rapid sequence. The wire feeder motor stops, so filler wire no longer advances into the arc. The power source cuts the welding current (or ramps it down if you have advanced controls), and the arc extinguishes.
At that point, the weld pool—still liquid and superheated—starts losing heat rapidly to the surrounding base metal and the air.
Because the edges of the pool cool and solidify first, the center often pulls inward as it freezes, creating a depression called a crater. In materials with high shrinkage rates, like aluminum, this contraction sets up tensile stresses that can tear the metal and form a crater crack.
On steel, you might see porosity instead if the shielding gas stops too soon, leaving the hot weld exposed to oxygen and nitrogen.
The shielding gas continues for a few seconds after the arc stops (post-flow), which is critical. Without it, the cooling crater can oxidize, forming surface defects or inclusions. Burnback can also occur here if your machine settings are off—the arc climbs up the wire and fuses it to the contact tip before everything shuts down cleanly.
In my experience, the first few seconds after stopping the arc are make-or-break. The pool is still fluid enough to manipulate with technique, but it solidifies fast. Rush it, and you pay later.
Why Craters Form and How They Turn into Cracks
Craters are basically the fingerprint of how the weld pool solidifies. When the arc is running, the pool is kept molten and fed with fresh wire. Stop the heat and wire input abruptly, and surface tension plus rapid cooling from the base metal pulls the pool into a saucer shape. The center solidifies last, creating a low spot.
In high-shrinkage metals like aluminum or certain tool steels, these stresses concentrate and crack. I’ve repaired plenty of aluminum truck beds where the welder just stopped cold—the crater cracked straight through because the material cools so quickly it doesn’t have time to feed filler into the depression.
Even on mild steel, a deep crater becomes a notch that concentrates stress. Under vibration or load, cracks propagate from there. The fix is simple in concept but takes practice: you want the final bead profile at the stop to be flat or slightly convex, not concave.
Common causes I see:
- Releasing the trigger while still moving forward at full speed.
- No backstep or crater-fill function.
- Insufficient post-flow, letting air hit the hot metal.
- Wrong voltage or wire speed that leaves a skinny, fast-freezing puddle.
The Critical Role of Shielding Gas Post-Flow
Post-flow is your invisible bodyguard for the cooling crater. Once the arc stops, the gas keeps flowing to blanket the hot weld and prevent oxidation and porosity. Without enough post-flow, you’ll often see a gray, sugary-looking surface or tiny pinholes right at the end of the bead.
Typical recommendations I’ve used successfully in the shop:
- Mild steel with C25 (75% argon/25% CO₂): 8–12 seconds.
- Stainless steel with tri-mix: 10–15 seconds.
- Aluminum with pure argon or argon/helium: 15–20+ seconds, sometimes longer on thick sections.
Many modern USA machines like Miller Millermatic or Lincoln PowerMIG series let you dial this in directly. On older transformer machines, you might have to count it out manually while holding the gun steady over the crater.
Pro tip: Don’t pull the gun away immediately. Keep the nozzle in place for the full post-flow time, angled slightly to keep the gas coverage tight. In drafty shops, bump the flow rate or use a larger nozzle to maintain protection.
Machine Settings That Control a Clean Arc Stop
Your welder’s end-of-weld controls make a huge difference. Here’s what actually matters on the shop floor:
Burnback time controls how long the power stays on after the wire feeder stops. Too little and the wire sticks in the puddle. Too much and it burns back into the contact tip, ruining it. I usually start at mid-range (around 50–70 on a 0–100 scale) and adjust based on wire diameter and voltage.
Crater fill (or downslope) gradually reduces current and/or wire speed at the end instead of cutting off abruptly. This lets you add a bit more metal to fill the depression. Synergic machines often have preset crater programs that work well once you dial in your base parameters.
Post-flow we already covered—treat it as non-negotiable.
Inductance or arc control affects how fluid the puddle is. Higher inductance gives a softer, wetter arc that fills craters more easily but can increase spatter if overdone.
For common US setups with 0.035″ wire on mild steel:
| Thickness | Voltage | Wire Feed Speed (ipm) | Approx. Amperage | Recommended Post-Flow |
|---|---|---|---|---|
| 1/8″ | 18–20V | 200–300 | 120–160A | 8–10 sec |
| 1/4″ | 20–22V | 300–400 | 180–220A | 10–12 sec |
| 3/8″+ | 22–24V | 400–500 | 220–280A | 12–15 sec |
Always test on scrap first. Every machine and gas mix behaves a little differently.
Step-by-Step: How I Stop the Arc in the Shop
Here’s the exact sequence I teach every new welder:
- As you approach the end of the joint, slow your travel speed slightly to build up a little extra metal in the puddle.
- Stop forward movement completely while keeping the arc on.
- If your machine has crater fill, trigger it now (many have a 4T or special button mode).
- Backstep ¼ to ½ inch over the already-welded bead while adding a quick burst or two of wire. This fills the crater and buries any potential crack starter.
- Release the trigger but hold the gun perfectly still over the crater for the full post-flow duration.
- Once the gas stops and the metal has visibly dulled in color, move the gun away.
On aluminum, I often do two or three quick trigger taps at the end—each shorter than the last—to stack small dabs of filler into the crater as it forms.
For short stitch welds or tacks, the backstep is especially useful because the crater makes up a larger percentage of the total weld.
Material-Specific Considerations
Mild steel is forgiving. Good C25 coverage and a proper backstep usually solve most issues. Watch for excessive spatter at the stop, which usually means voltage is a touch high or inductance too low.
Stainless steel needs more post-flow because the material stays hot longer and is prone to sugaring (oxidation). Use tri-mix gas and keep heat input controlled—stainless distorts easily, so a clean stop prevents extra grinding later.
Aluminum is the toughest. High thermal conductivity means the puddle freezes fast and cracks easily. Pure argon or argon-helium mixes, larger wire (0.047″ when possible), and generous post-flow are essential. The backstep technique is almost mandatory on anything thicker than 1/8″.
I’ve had students struggle with aluminum until they learned to “button off”—hold the gun still after releasing the trigger and watch the puddle color. When it starts to lose its bright sheen, hit the trigger for half a second to drop one more dab of filler.
Common Mistakes Beginners and Pros Still Make
The biggest one I see? Treating the stop like an afterthought. Welders focus so much on the middle of the bead that they rush the end.
Another frequent error is yanking the gun away too soon, which exposes the crater to air and causes porosity or oxidation. Or setting post-flow too short because “it wastes gas.” In reality, the cost of a little extra gas is nothing compared to cutting out and rewelding a defective crater.
Pros sometimes get sloppy on long production runs and stop relying on muscle memory instead of deliberate technique. I’ve caught myself doing it when tired—then the inspector finds the crack.
Wrong wire diameter or poor joint prep also hurts. A tight fit-up with clean metal gives you a more stable puddle to work with at the stop. Rusty or oily material makes the puddle behave unpredictably.
How GMAW Arc Stops Compare to SMAW (Stick Welding)
In stick welding (SMAW), you burn the electrode down to a stub, break the arc, chip slag, and restart if needed. The crater issue exists there too, but you deal with it differently—often by whipping the rod or using a slight pause at the end.
GMAW is continuous, so you don’t have the slag protection or stub to manage, but you also don’t get natural pauses. The big advantage in MIG is the ability to use machine controls like crater fill and post-flow, which SMAW doesn’t have. The disadvantage is that MIG craters are more exposed because there’s no slag blanket.
I’ve switched between processes on the same job many times. On repair work with limited access, I might start with stick for the root and finish with MIG for the cap—making sure the MIG stop ties in cleanly to avoid a weak point.
Joint Preparation and Technique Tips for Reliable Stops
Clean metal is non-negotiable. Grind or wire-wheel the stop area especially well—any mill scale or contamination will worsen defects when the puddle freezes.
Maintain a consistent gun angle throughout, then at the very end, tilt the gun back a bit (more drag angle) to push extra metal into the crater.
For out-of-position welds (vertical-up or overhead), slow everything down and use shorter post-flow bursts if the puddle wants to sag.
On thin material, reduce voltage slightly near the end so the puddle doesn’t burn through when you pause.
Always have a scrap piece nearby to test your stop technique before committing to the actual part.
Final Takeaway from the Shop Floor
After years of welding everything from race car chassis to heavy equipment repairs, I’ve learned that the last half-inch of every bead deserves as much attention as the first. A clean, filled crater with proper gas protection turns a good weld into a great one that passes every test and holds up in service.
You’re now equipped to look at that trigger release moment differently—not as the end of the weld, but as one of the most critical parts. Pay attention to it, dial in your machine, and practice the backstep until it becomes automatic.
On any critical weld, end on a tack or previously welded section whenever possible. It gives you a built-in crater filler and eliminates the weak spot entirely.
FAQ
How do I prevent crater cracks in MIG welds?
Backstep at the end—reverse direction ¼ to ½ inch and add filler before releasing the trigger. Use your machine’s crater fill function if available, and always run full post-flow. On aluminum, this is essential.
What post-flow time should I use for different materials?
Mild steel: 8–12 seconds. Stainless: 10–15 seconds. Aluminum: 15–20+ seconds. Adjust upward in windy conditions or on thicker/hot parts.
Why does my wire burn back into the contact tip when I stop welding?
Burnback time is set too high or voltage is excessive relative to wire speed. Start in the middle of the range and test. Also check contact tip condition and stickout.
Can I fix a crater after the weld has cooled?
Yes, but it’s more work. Grind out the crater and any cracks completely, then reweld with proper technique. Prevention is always better and faster.
What’s the difference between burnback and crater fill settings?
Burnback controls how long power stays on after wire feed stops to prevent sticking and prepare for the next start. Crater fill gradually reduces current/wire speed to add metal and fill the depression as the puddle cools.
